(1) Field of the Invention
This invention relates to a radiographic apparatus having a radiation source and a radiation detector, and more particularly to a radiographic apparatus, which can radiograph the lung field and abdomen, and carry out a subtraction process for emphasizing bone tissues and soft tissues.
(2) Description of the Related Art
A radiographic apparatus is installed in a medical institution for acquiring radiological images of patients. Such a radiographic apparatus has a radiation source for emitting radiation, and a radiation detector for detecting the radiation (Japanese Unexamined Patent Publication No. 2004-057559, for example).
A construction of a conventional radiographic apparatus will be described. A conventional radiographic apparatus includes a radiation source, a radiation detector, an image generating unit and a superimposing unit. Such radiographic apparatus is constructed to carry out two radiographing operations to generate a single subtraction image based on a difference between the two fluoroscopic images obtained from the radiographing operations.
The output energy of the radiation source is different between the first radiographing operation and second radiographing operation. The radiation source provides a high energy output for the first radiographing operation, and a low energy output for the second radiographing operation. In both operations, images of bone tissue and soft tissue of the patient are acquired.
In the first radiographing operation, the degree of radiation transmission is different between the bone tissue and soft tissue. In the second radiographing operation also, the degree of radiation transmission is different between the bone tissue and soft tissue. And there is no agreement between the way in which the radiation transmission is different in the first radiographing operation and that in the second radiographing operation. There occurs a phenomenon in which, for example, although the first radiographing operation with the high energy radiation produces a considerable difference in pixel value between a portion of bone tissue and a portion of soft tissue, the second radiographing operation with the low energy radiation produces little difference in pixel value between the portion of bone tissue and the portion of soft tissue. When a difference is determined between the two images having such characteristics, a subtraction image emphasizing the bone tissue can be acquired, with the portions of soft tissue reflected in the two images canceled, and the portions of bone tissue appearing more clearly.
When the two images are superimposed after a tone correction, a subtraction image showing the portion of soft tissue more clearly can be acquired.
A conventional radiographic apparatus for radiographing the chest and abdomen includes a radiation source, a radiation detector and an image generating unit. Such radiographic apparatus is constructed to carry out still image radiography by emitting still (single-shot) radiation to generate still images showing the lung field of a patient.
The lungs of the patient always continue moving due to respiration of the patient. For observing the lungs, it is desirable to carry out still image radiography in a state of maximum lung expansion. Then, at a time of radiography, the patient is instructed to inhale air and suspend respiration temporarily in that state.
For observing the abdomen, it is desirable to carry out still image radiography in a state of maximum lung contraction. Then, at a time of radiography, the patient is instructed to exhale air and suspend respiration temporarily in that state.
However, the conventional radiographic apparatus have the following problems.
The conventional radiographic apparatus has a problem that the patient can move during the time between the first radiographing operation and the next radiographing operation, thereby blurring a subtraction image. When there is a difference between a patient's position imaged in the first radiographing operation and a patient's position imaged in the second radiographing operation, a subtraction image will be acquired from a superimposition of the images having the patient's positions shifted from each other. Therefore, the subtraction image acquired will become blurred.
Even if the patient is instructed not to move at the time of radiography, the interior of the patient constantly keeps moving due the patient's heartbeat. When radiography is carried out without considering this movement, a clear subtraction image cannot be acquired.
Then, conventionally, a heart rate is checked by electrocardiography to grasp the cycles of the interior of the patient assuming the same shape, and radiography is carried out for each cycle. That is, a first radiographing operation is started when the heart expands to the full, and a second radiographing operation is started when the heart is in the same state again.
A subtraction image acquired by such a method is blurred also. This is because, when compared at the end of each radiographing operation, the states of the heart are not in agreement. According to the conventional construction, the first and second radiographing operations are the same only in the timing of starting radiation emission, and there is no guarantee that the first and second radiographing operations are carried out when the heart is in the same shape.
The conventional radiographic apparatus needs electrocardiographic measurement, and its operation is complicated. Such construction is not desirable for a large medical institution, for example, where many fluoroscopic images must be acquired with a single apparatus.
The radiographic apparatus for radiographing the chest and abdomen has a problem that it cannot perform radiography suited to the purposes of examination when radiographing patients unable to cope with instructions on respiration of the patient, such as infants, old patients, and patients with artificial respirators.
When such a patient cannot be radiographed in a state of suspending respiration, the operator needs to carry out still image radiography by choosing the right timing of respiration of the patient. That is, the operator visually observes a swelling of the body surface of the patient, and carries out still image radiography at the moment of the patient finishing inhalation or exhalation.
Still images acquired in such a state cannot necessarily be said ideal for diagnosis. It is difficult to recognize visually an inhaling state of the patient. In addition, it is difficult to operate the radiographic apparatus to take a still image immediately after the operator decides to carry out radiography. An expanding state of the lungs at the time of radiography is not necessarily a desired one.
There is a conventional construction which can carry out still image radiography synchronously with respiration by using a distance sensor for monitoring approach and withdrawal of the chest of a patient. With this construction, however, it is difficult to grasp the cycle of respiration accurately when the patient wears thick clothes or when the respiration of the patient is weak. After all, a situation will occur where a swelling state of the lungs at the time of radiography is not a desired on.
Unless a swelling state of the lungs is constant at the time of radiography, a problem will arise when observing variations in the disease condition of the patient. That is, since the swelling state of the lungs of the patient is different for each examination, it will be unclear whether the difference between still images upon comparison is due to variations in the disease condition or the difference is due to a difference in the swelling state of the lungs.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus which can acquire clear images in a simple way.